Investigators in the Divisions of Clinical Pharmacology and Cardiology at Duke University Medical Center have used the Rankin General Clinical Research Center to study patients with atrial fibrillation and other common disorders of the heart rhythm. Atrial fibrillation is the most common disorder of the heart rhythm requiring treatment with antiarrhythmic drugs, and it accounts for over one-third of all U.S. hospitalizations for arrhythmias. Patients with atrial fibrillation have their usually normal heart rhythm replaced by periods of rapid, irregular heart beating that may cause a sense of pounding in the chest, shortness of breath, chest pain, dizziness, or loss of consciousness. These periods may last for minutes or hours, or they may last indefinitely unless acute treatment is given by a physician. Studies conducted on the Rankin General Clinical Research Center have concentrated on elucidating the mechanism, clinical course, and optimal treatment of atrial fibrillation and related disorders of the heart rhythm. These studies have focused on measuring the frequency of spontaneous occurrence of atrial fibrillation and paroxysmal supraventricular tachycardia. Among 150 patients studied who had their antiarrhythmic drug therapy withdrawn, one-half had a spontaneous, symptomatic recurrence in about 3 weeks; in fact, about 1 in 5 had a symptomatic recurrence of their arrhythmia in less than 1 day. The average heart rate during recurrences was 132 beats/min for patients with atrial fibrillation and 198 beats/min for patients with paroxysmal supraventricular tachycardia. Among the patients with atrial fibrillation, about 1 in 9 had a second arrhythmia, atrial flutter, recorded along with fibrillation. The frequency of asymptomatic arrhythmias was approximately 12 times the rate of symptomatic arrhythmias. Curiously, quality of life was not affected by the frequency of arrhythmia occurrence. Similar methods were used to study patients with ventricular tachycardia who had an implanted cardioverter defibrillator (ICD). The frequency of ICD shocks in this population provided an estimate of the occurrence of ventricular tachycardia. About 1/3 of the patients had an episode of ventricular tachycardia within 1 month. Results of these studies are important for designing clinical trials of chronic antiarrhythmic drug therapy.